Unit test help me find bug in class today

As always, GUI has few unit tests because limited time frame, but unit test give me a good lesson today:

- I define a new extension point in Eclipse, write a couple of classes which allow me to process those extensions easily, then I write a very very simple unit test class in case somebody will shout at me, but guess what happen? that simple unit test help me find a deep bug in my code, I believe it's not easy to find that bug without detail code review, maybe it can be found during runtime, but unit test find it with no more than 10 lines. Cool unit test!